The present invention relates to shear separation methods and systems and, more particularly, to shear separation methods and systems wherein microfiltration, ultrafiltration, diafiltration, or concentration can be achieved.
Separation methods and systems, such as those employing filters, typically are employed to separate one or more components or substances of a fluid from other components or substances in the fluid. As used herein, the term xe2x80x9cfluidxe2x80x9d includes liquids, gases, and mixtures and combinations of liquids, gases and/or solids. Conventional separation processes include a wide variety of common processes, such as classic or particle filtration, microfiltration, ultrafiltration, nanofiltration, reverse osmosis (hyperfiltration), dialysis, electrodialysis, prevaporation, water splitting, sieving, affinity separation, purification, affinity purification, affinity sorption, chromatography, gel filtration, bacteriological filtration, and coalescence. Typical separation devices and systems may include dead end filters, cross-flow filters, dynamic filters, vibratory separation systems, disposable filters, regenerable filters including backwashable, blowback and solvent cleanable, and hybrid filters which comprise different aspects of the various above described devices.
Accordingly, as used herein, the term xe2x80x9cseparationxe2x80x9d shall be understood to include all processes, including filtration, wherein one or more components of a fluid is or are separated from the other components of the fluid. The terms xe2x80x9cfilterxe2x80x9d, xe2x80x9cseparation mediumxe2x80x9d, and xe2x80x9cpermeable membranexe2x80x9d shall be understood to include any medium made of any material that allows one or more substances in a fluid to pass therethrough in order to separate those substances from the other components of the fluid. The terminology utilized to define the various substances in the fluid undergoing separation and the products of these processes may vary widely depending upon the application, e.g., liquid or gas filtration, and the type of separation system utilized, e.g., dead end or open end systems; however, for clarity, the following terms shall be utilized. The fluid which is input to the separation system shall be referred to as process fluid and construed to include any fluid undergoing separation. The portion of the fluid which passes through the separation medium shall be referred to as permeate and construed to include filtrate as well as other terms. The portion of the fluid which does not pass through the separation medium shall be referred to as retentate and construed to include concentrate, bleed fluid, as well as other terms.
While many separation applications are quite routine, the separation of relatively small particles or substances from fluids requires separation protocols able to achieve a precise separation size (resolution) with minimal fouling (e.g., clogging with the small particles). This is particularly the situation when separating proteins (natural or recombinant) and other components from process fluids such as milk or products derived from milk (e.g., skim milk, whey, etc.).
Milk contains, among other things, fats, proteins (casein and a variety of other proteins such as xcex2-lactoglobulin, xcex1-lactalbumin, serum albumin, and immunoglobulins), salts, sugar (lactose), and various vitamins (such as vitamins A, C, and D, along with some B vitamins) and minerals (primarily calcium and phosphorus). The composition of milk varies with the species, breed, feed, and condition of the animal from which the milk is obtained. Moreover, a wide variety of milk or whey proteins are employed as functional and nutritional ingredients in bakery products, pasta, confections, beverages, meats, and other food products. In addition, milk has proven a valuable source of biologically or medically important products. For example, it is possible to obtain antibodies by vaccinating lactating animals and collecting antibodies from their milk (see, e.g., U.S. Pat. Nos. 5,260,057 (Corcle et al.) and 3,128,230 (Heinbach et al.)). Moreover, many species of animals have been genetically engineered to express recombinant proteins in milk. See, e.g., Gordon et al., Biotechnology, 5(11), 1183-87 (1987) (mice); Ebert et al., Biotechnology, 12(7), 699-702 (1994) (goats); Lee et al., J. Control. Release, 29(3), 213-21 (1994) (dairy cows); Limonta et al., J. Biotechnol., 40(1), 49-58 (1995) (rabbits); Clark et al., Biotechnology, 7(5), 487-92 (1989) (sheep).
Examples of such recombinant proteins are peptide hormones (e.g., growth hormones (Archer et al., Proc. Nat. Acad. Sci. USA, 91(15), 6840-44 (1994)), tissue plasminogen activator (tPA) (Ebert et al., supra), etc.), blood coagulation factors or subunits of them (e.g., factors VIII and IX (Clark et al., supra)), anticoagulation factors or subunits of them (e.g., anti-thrombin III and human protein C), other blood proteins (e.g., serum albumin (Barash et al., Mol. Repro. Dev., 45(4), 421-30 (1996)), beta-globin, xcex11-antitrypsin (Archibald et al, Proc. Nat. Acad. Sci. USA, 87(13), 5178-82 (1990)), proteins for foodstuffs, enzymes, and other proteins (e.g., collagen, cystic fibrosis transmembrane conductance regulator (CFIR), antibodies, etc.). See, e.g., U.S. Pat. No. 4,873,316 (Meade et al.), U.S. Pat. No. 5,589,604 (Drohan et al.), and U.S. Pat. No. 5,476,995 (Clark et al.). Secretion of recombinant proteins into the milk of transgenic animals is an efficient method of producing such proteins; concentrations approaching 10 g/I have been reported.
Commercially produced milk commonly undergoes pasteurization to mitigate bacterial growth and homogenization to improve fat dispersion stability. Moreover, in the commercial processing of milk products, it is desirable in certain instances to remove as much fat as possible from the milk products.
Conventional milk processing heretofore has involved the use of mechanical separation (centrifugation), evaporation/crystallization, steam injection, electrodialysis, reverse osmosis, ultrafiltration, gel filtration, diafiltration, and/or ion exchange chromatography. For example, whey typically is subjected to mechanical separation (e.g., centrifuged) to remove fat, condensed via evaporation to increase solids content, and then spray dried or used for lactose crystallization. After desludging, the residual concentrate is dried, which yields whey powder containing about 11-14% protein (which usually is denatured, particularly during the evaporation/condensation step). The whey powder can be subjected to electrodialysis to remove ash and thereby prepare demineralized whey powder. Alternatively, the whey powder can be subjected to reverse osmosis to remove water, thereby obtaining whey powder containing about 12-15% protein. Such a whey powder can be subjected to ultrafiltration or gel filtration to remove further ash and lactose and thereby obtain a whey protein concentrate containing about 30-50% protein, which, in turn, can be subjected to diafiltration or ion exchange chromatography to remove yet more ash and lactose so as to obtain whey protein concentrates containing about 50-90% protein.
Such conventional processing methods carry with them many disadvantages, such as long processing times, high costs, and poor or inconsistent component fractionation. Moreover, it is often difficult to separate a recombinant protein from fluids such as milk by these methods without denaturing or damaging the protein, and it is also difficult to separate different proteins and particles of interest within milk or other fluids. Many of these difficulties are attributable to the aforementioned problems attendant with separating relatively small particles from fluids, namely poor resolution and filter fouling.
One advancement greatly reducing filter fouling is to employ separation methods and systems generating a shear layer at the surface of a filter. A layer of fluid which is adjacent to the surface of a filter and which is in a state of rapid shear flow parallel to the surface of the filter tends to minimize fouling of the filter by sweeping contaminant matter in the process fluid from the filter. Generally two such technologies can be used for developing a shear layer: cross flow and dynamic filtration. In cross flow systems, high volumes of fluid are driven through passages bounded by the filter surface and possibly the inner surface of the filter housing, thereby creating the necessary shear. Simply stated, process fluid is pumped across the upstream surface of the filter at a velocity high enough to disrupt and back mix the boundary layer. In dynamic filter systems, the necessary shear is created by motion of one or more surfaces that can be provided for that purpose (e.g., the filter surface, the filter vessel, or any contained discs, impellers, etc.). Two widely used configurations are cylinder devices and disc devices. Unlike cross flow filtration systems, the shear created in dynamic filtration systems at the fluid interface is substantially or nearly independent of any cross flow fluid velocity. Traditional cross-flow filtration systems generate shear generally between about 5,000 secxe2x88x921 and about 10,000 secxe2x88x921, while shears generated by dynamic filtration systems are between about 100,000 secxe2x88x921 and about 500,000 secxe2x88x921.
While dynamic and traditional cross-flow filtration systems can achieve reduced fouling, the size or molecular weight cutoff of the particles of interest is controlled by the separation medium. In both systems, the actual separation or filtering action is effected by the separation medium, the pores of which are sized to remove or separate the particles of interest. Particles larger than the pores are unable to pass through the separation medium while particles smaller than the pores readily pass through the medium. Due to the fouling characteristics of a process fluid and the inherent difficulties in engineering filter media with uniform and predefined pore sizes, high resolution separation of relatively small particles (e.g., molecular weight particles) has been exceedingly difficult using dynamic and cross flow filtration systems.
To address these drawbacks, there is a need for improved means for separating small substances from a solution or a suspension, even a highly fouling solution or suspension. In particular, there is a need for means of effectively concentrating particles of a given molecular weight (e.g., specific proteins), thereby achieving fractionation of such solutions or suspensions. The present invention provides a reliable and efficient means for the separation of small particles or substances (e.g.,molecular size particles) from a variety of fluids (e.g., solutions, suspensions, emulsions, etc.), especially milk products.
The present invention provides shear separation systems and methods for treating a process fluid to separate substances having a size less than a predetermined separation size from substances having a size greater than the separation size. Permeate flow through a permeable membrane establishes a drag force acting on substances upstream of the permeable membrane, and a shear rate is created at the surface of the permeable membrane to establish a lift force acting on substances upstream of the permeable membrane. The balance of the drag force and lift force effects the predetermined separation size in that the balance of forces retards the transmembrane passage of substances larger than the separation size yet allows substances smaller than the separation size to pass through the permeable membrane. Shear separation systems and methods embodying the invention can be employed to separate or concentrate from a process fluid substances larger or smaller than the predetermined separation size by collecting them from the retentate or permeate.
The present invention effects the separation or concentration of relatively small particles or substances, such as proteins and other biological molecules, far more effectively and with much greater flexibility than conventional systems or methods. Therefore, the separation methods and systems of the present invention are useful for the efficient separation of substances from a wide variety of fluids. For example, the present invention can treat milk products to reduce the bacteria and fat therein and/or to recover proteins therefrom, even fractionating such fluids. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the drawings and the detailed description outlined below.